The invention relates to a method and an apparatus for the tanking up or refuelling of a pressure container with a gaseous medium. Furthermore the invention relates to tanking apparatus and tanking installations operated using the method and/or including the apparatus. The invention relates in particular to the tanking of motor vehicles operated with gas.
It is known to use a pressure container as a storage means for a gaseous medium. The stored gas is used in a process, for example in a combustion process, so that the pressure container is partly or fully emptied in the course of time and must be directed to a tanking or filling installation in order to be filled up again. Pressure containers as storage means for gaseous media are gaining increasing significance for motor vehicles because they make it possible to use alternative gaseous fuels, such as for example methane, ethane, propane, hydrogen or gas mixtures such as naturally occurring natural gas for the operation of the vehicle. The use of compressed natural gas for the operation of motor vehicles is nowadays increasingly gaining in significance, in particular in countries which have rich natural gas deposits. Motor vehicles can moreover be converted in a very simple manner to operation with natural gas and, when burned, natural gas is characterized by a low pollution combustion in comparison to petrol or diesel, and thus by a reduced environmental burden.
In order to keep the volume of the pressure container at a reasonable size with respect to the motor vehicle it is customary to store natural gas and other gaseous media under a relatively high pressure, for example at 200 bar, related to a reference temperature of 15.degree. C.
The acceptance of such an energy store can be improved amongst motor vehicle owners if one succeeds in tanking the pressure container in a very simple manner comparable to tanking with gasoline. A tanking with natural gas involves substantially more sources of danger in comparison to gasoline because the gas is transferred at high pressure into the pressure container of the vehicle, so that special coupling parts, valves and safety devices are necessary. Furthermore, the aim is to minimize the danger of fire or of an explosion and in particular account has to be taken of the fact that changes of the ambient temperature influence the pressure of the natural gas in the pressure container, so that the pressure could, in the most unfavorable case, rise to an impermissibly high value. It is an extremely demanding operation to fill a pressure container by tanking to approximately 100%, that is to say to fill the pressure container to a preset nominal pressure of for example 200 bar, because the pressure in the pressure container depends on the temperature of the gaseous medium.
A gas tanking apparatus for motor vehicles is known from EP-O-356-377 in which the gas is directly taken from a gas line and compressed with a compressor and directly supplied to a pressure container of a vehicle via a tanking hose and a coupling. The gas pressure is monitored by a pressure sensor which is arranged between compressor and tanking hose. As the tanking procedure takes place very slowly it can be assumed, with the known gas tanking apparatus, that the pressure measured in the gas tanking apparatus between the compressor and the tanking hose corresponds approximately to the pressure in the pressure container of the motor vehicle. The gas tanking apparatus furthermore has a temperature sensor for determining the outer temperature which permits, as a result of a normed association between pressure and temperature, a permissible maximum pressure in the pressure container for the gaseous medium such as natural gas to be computed as a function of the external temperature. The gas tanking apparatus terminates the tanking procedure as soon as the pressure sensor has reached the preset maximum pressure.
As a tanking procedure normally lasts for several hours a temperature compensation takes place so that the temperature of the gas in the pressure container corresponds approximately to the environmental or ambient temperature. A disadvantage of this known gas tanking apparatus is the fact that a complete tanking procedure requires a long period of time, normally several hours. This kind of gas tanking apparatus is for example used for filling the pressure container of motor vehicles overnight.
A further gas tanking apparatus which permits a rapid tanking of a pressure container of a motor vehicle, comparable with tanking with gasoline, is known from WO-93/00264. This apparatus also measures the external temperature in order to determine a permissible maximum filling pressure p.sub.z corrected in accordance with the environmental temperature. As the gas tanking apparatus presses the gas very rapidly into the pressure container of the motor vehicle a pressure drop develops in the supply lines, valves, etc., so that the pressure measured at the output of the gas tanking apparatus of a pressure sensor no longer corresponds to the pressure of the gas in the pressure container. During the filling of gas into the pressure container the inner pressure of the pressure container is thus not detectable. Nevertheless, it must be ensured that the pressure container is filled with gas and that the maximum permitted pressure p.sub.z is not exceeded. The cited publication uses the following method for the tanking of the pressure container:
The pressure container of the motor vehicle is connected via a tanking hose with a gas tanking apparatus, the environmental temperature is measured, from this a corresponding switch off pressure p.sub.z is computed, a small quantity of gas is forced into the pressure container in order to open the non-return valve and thereupon the mass flow of the gas is interrupted. As the non-return valve of the pressure container is now open the pressure p.sub.vo in the pressure container can be determined with a pressure sensor which is located in the gas tanking apparatus, since a pressure compensation takes place between the gas tanking apparatus and the pressure container. Thereafter a specific quantity of gas m.sub.1 is pressed by the gas tanking apparatus into the pressure container, the mass flow of the gas is again interrupted and the pressure p.sub.v1 acting in the pressure container is measured by the pressure sensor in the gas tanking apparatus. Based on these two measurement points the volume V of the pressure container is first calculated and then the required mass of gas which is to be supplied in order to fill the pressure container to the predetermined pressure p.sub.z. Thereafter the gas forwarding means is set in operation, the required quantity of gas is measured and the tanking procedure is terminated as soon as the calculated quantity of gas has been discharged from the gas tanking apparatus into the pressure container. A substantial disadvantage of this tanking procedure is the fact that it is necessary in a first step to compute the volume of the pressure container and that it is necessary in a second step to calculate the required mass of gas from the volume.
The tanking method uses the known gas equations for calculating the volume of the pressure container. EQU For an ideal gas: pV=(m/M)RT (1) EQU For a real gas: pV=Z(m/M)RT (2)
wherein
p=pressure PA0 V=volume PA0 m=mass of the gas PA0 M=molecular weight of the gas PA0 R=universal gas constant PA0 T=temperature PA0 Z=gas compression factor PA0 m.sub.1 =specific known mass of gas PA0 Z.sub.0, Z.sub.1 =gas compression factor at the measuring point 0 and the measuring point 1 PA0 R=universal gas constant PA0 T.sub.u =environmental temperature PA0 M=molecular weight of the gas PA0 P.sub.v0, p.sub.v1 =pressure at the measuring point 0 and measuring point 1 PA0 T0, T1=temperature measuring point 0 and measuring point 1. PA0 Z.sub.z =gas compression factor at the point p.sub.z.
The described measuring method detects the two pressures p.sub.v1, p.sub.v0 and also the mass m.sub.1. Taking account of a real gas the volume of the pressure container V can be calculated in accordance with the following formula: EQU V=(m.sub.1 Z.sub.1 R T.sub.u)/(M (p.sub.v1 -p.sub.v0 ((Z.sub.1 /Z.sub.0) (T.sub.1 /T.sub.0))))
with the following relationships applying:
After the volume V of the pressure container has been computed the required mass of gas m.sub.2 is calculated which is necessary to fill the pressure container to the calculated pressure p.sub.z. In doing so the gas equation is solved in accordance with the mass so that: EQU m.sub.2 =V M/R T.sub.u ((p.sub.z /Z.sub.z)-(p.sub.v1 /Z.sub.1))
wherein
This mathematical method for the calculation of the required mass of gas m.sub.2, which calculates the volume V of the pressure container in a first step and the required mass m.sub.2 of the gas in a second step, has various disadvantages. Thus it is not possible as a result of purely physical laws to determine the volume V precisely or with adequate accuracy as a result of the formulas that are used, because the values of some of the parameters that are used are not measurable and are thus not known for the following reasons:
1. The temperatures T.sub.0 and T.sub.1 relate to the temperature of the gas in the pressure container which cannot however be determined with the present method because the customary pressure containers of motor vehicles do not have any integrated temperature sensor. In this respect it can in particular not be assumed that T.sub.0 and T.sub.1 correspond to the environmental temperature. This is for the following reason: Natural gas is a real gas. If an almost empty pressure container is tanked then the known Joule-Thomson effect occurs which results in the temperature of the gas in the interior of the pressure container dropping rapidly with the initial powerful relaxation of a real gas, so that it is to be assumed that the temperature(s) T.sub.0 and/or T.sub.1 fall far below the environmental temperature. The situation is quite different when the pressure container is close to filled. Then the gas in the pressure container only relaxes fractionally so that the temperatures T.sub.0 and T.sub.1 only change a little. The temperatures T.sub.0 and T.sub.1 are thus strongly influenced by the initial pressure which is present in the pressure container at the start of tanking. PA1 2. Natural gas is a gas mixture which customarily consists of over 90% methane and other components such as ethane, propane, butane, nitrogen, carbon dioxide, etc. It is known that the composition of a natural gas drawn from a public gas supplier can fluctuate daily. This has the consequence that both the molecular weight of the gas M and also the gas compressibility factor Z may not be assumed to be constant.
The described method thus has the disadvantage that the volume V of the pressure container cannot be accurately calculated as a result of physical laws, and, for the same reasons, the additional mass m.sub.2 to be filled in is also not accurately computable. A decisive disadvantage of the known method is thus the fact that the additional mass m.sub.2 which is filled into the pressure container generates a pressure in the pressure container which can lie in a wide range of scatter about the desired pressure p.sub.z. Dangers can then arise when the pressure in the pressure container rises to substantially above the maximum permitted pressure p.sub.z. It is however also disadvantageous when the pressure in the pressure container remains below the maximum permitted pressure p.sub.z, because the pressure container could then take up more gas for a complete filling.
It is known that the heating or cooling of the gas in the pressure container, in particular when filling an empty tank, starts with a rapid dynamic process at the beginning of the tanking procedure. The gas relaxes during filling into the pressure container and thereby cools down quickly. At the same time a heat exchange takes place between the warmer wall of the pressure container and the gas. This highly dynamic process typically takes place during the first 30 seconds of the filling process. If the filling is interrupted then an equilibrium sets in again between the temperature of the outer wall of the pressure container and the gas. A disadvantage of the known method is thus the fact that for determining the two measurement points p.sub.v0, p.sub.v1 a relatively small specific mass of gas m.sub.1 must be supplied to the pressure container at the start of the tanking process. Directly after the supplying of the mass of gas m.sub.1 the pressure p.sub.v1 is measured and this value is afflicted with a large degree of uncertainty. The subsequent calculation of the volume V and also of the mass m.sub.2 is thus correspondingly subject to error.
A further disadvantage of the known method is the fact that the mass flow of gas is determined in accordance with the principle of the "sonic nozzle". For the precise measurement of the throughflowing mass flow a correspondingly high pressure drop of the gas across the "sonic nozzle" is necessary.
The pressure in the preceding storage tank must be kept at a correspondingly high value. The gas must be compressed to a higher degree in the preceding storage tank, which requires an increased energy requirement for the compression. Furthermore the elevated pressure in the storage tank leads, with an empty pressure container, to a very pronounced Joule-Thomson effect, so that the gas in the pressure container is cooled down to very low temperatures shortly after the start of tanking, which is why the danger exists that the water and methane crystallize to hydrates and these crystalline structures coat or block the supply lines, coupling points or valves.